And it's proving to be a lucrative racket for roaming gangs, who are mainly comprised of Eastern European criminals.

The gangs are scouring the lakesides around the country and the banks of the river Shannon for boats moored in quiet places with little security.

"This is the best month of the year for the marine mobs," said Det Sgt Finbarr Garland of the Garda's stolen vehicles unit.

"Boat enthusiasts are usually preparing for one more outing before they put them away for the winter and can be relatively slack about security during September."

The gangs seek out targets in secluded areas and strike at night, putting the engines into vans and then shipping them to mainland Europe for a black market sale.

The most popular engine type for the thieves is an 8hp; there is a stolen list of 152 for the past year. These include 50 Yamahas and 40 Hondas, which seem to be favoured by the gangs. The haul is worth about €600,000.

Many of the owners are not aware of the identification details of their boats, apart from the make and colour, and this causes complications when gardai attempt to return recovered items to their owners.

Officers say a poster campaign with the Inland Waterways authorities and Crimestoppers has resulted in some successes, but they warn owners to remain vigilant for the rest of the season.